The invention set forth in this specification pertains to new and improved temperature indicating devices. More specifically it pertains to devices of this type which are utilized in indicating whether or not the device has been exposed to a predetermined temperature.
Devices of this type are employed in a wide variety of diverse applications so as to indicate whether or not something has been heated to or cooled to a predetermined temperature and so that this can be determined regardless of the temperature at the time a device of this type is inspected. These devices are not thermometers, but are more in the nature of recording structures designed to record whether or not specific temperatures have been achieved. Devices as are indicated may be utilized in determining whether or not a shipment of a temperature sensitive material has been heated to an undesired extent or has been cooled to an undesired extent during shipment from one location to another and/or during storage.
In the past a number of different methods have been employed for indicating whether or not certain specific temperatures have been achieved or reached. Devices such as known specialized types of recording thermometers and the like are considered to be undesirably expensive for use in determining the temperature "history" of a specific item or commodity. In some applications it is considered undesirable to utilize various types of coatings which change in color because of either a chemical reaction or a physical action caused by a temperature change. There are a number of reasons for this which are unimportant to an understanding of the present invention. These coatings which undergo a reversible color change are unusable for recording a past temperature after a return to an initial temperature. Coatings which undergo an irreversible color change can undergo such a change prior to being utilized in a desired application, and, of course, when this occurs they cannot be utilized for their intended purpose.
It is also known to utilize various different structures in which a temperature change causes a rapid or significant movement in a spring type bimetallic or similar element so as to indicate whether or not a specific temperature has or has not been developed. Some of these devices have been constructed in such a manner that a temperature change from an initial value to a predetermined value to be monitored is not indicated after the temperature of the device is changed back from the predetermined value to the initial value. Other of these devices are considered undesirable because of the amount of space occupied or required by the spring or spring element and/or the complexity of the spring element. Further, some of these devices are considered undesirable because they do not provide an easily observed indication as to whether or not they have been heated or cooled to a specific predetermined temperature. Certain of these devices are considered to be unnecessarily complex and expensive from a mechanical standpoint. Questions have also been raised as to the reliability of some of these devices. These latter factors are believed to be quite important. For a temperature indicating device to be commercially acceptable in a number of different applications, such a device must be simple enough so that it can be inexpensively produced on a mass production basis and must be highly reliable to the extent that there is virtually no chance of such a device not performing satisfactorily.